Manganese dioxide is known and widely used as a solid electrolyte in electrolyte capacitors. Such capacitors are conventionally formed by first anodizing a valve-metal anode (e.g., tantalum) to form a dielectric oxide coating, and thereafter immersing the oxide-coated anode in an aqueous solution of manganese nitrate. After a sufficient period of time, the wet anode is heated to cause pyrolytic decomposition of the manganese nitrate to manganese dioxide. To achieve the desired thickness of the solid electrolyte, the steps of immersion and heating are often repeated multiple times. Unfortunately, one problem with conventional manganizing techniques is that the thickness of the resulting manganese dioxide is often greater at certain locations of the anode (e.g., edges), which can lead to poor electrical performance. Various techniques have been employed in an attempt to address these problems. For example, surfactants have been employed in the manganese nitrate solution to substantially reduce its surface tension and improve the wettability of the surface of the oxide-coated anode. One such surfactant is Erktantol® NR (Tanatex Chemicals BV), which is a nonionic fatty alcohol polyglycol ether. Likewise, U.S. Pat. No. 4,302,301 to Tierman describes various other nonionic surfactants that can be employed in the manganizing solution, such as nonylphenoxypoly-(ethyleneoxy)ethanol (Igepal CO-630); isooctylphenoxy-polyethoxyethanol (Triton X-100), benzyletheroctylphenol-ethylene oxide condensate (Triton CF-10), and 3,6-dimethyl-4-octyne-3,6-diol (Surfynol 82).
Although the addition of surfactants may provide some benefits, significant problems nevertheless remain. For example, the capacitors may still exhibit a relatively large loss in capacitance when wet and a high leakage current. This problem is particularly evident when the valve metal powder used to form the anode has a high specific charge—i.e., about 70,000 microFarads*Volts per gram (“μF*V/g”) or more. Such high “CV/g” powders are generally formed from particles having a small size and large surface area, which results in the formation of small pores between the particles that are difficult to impregnate with the manganese nitrate solution. The difficulty in impregnating such small pores leads to the formation of manganese dioxide particles that are large in size and irregularly shaped. These particles do not adhere well to the dielectric coating and are unable to achieve good surface coverage, which leads to poor electrical performance of the capacitor.
As such, a need currently exists for an improved electrolytic capacitor containing a manganese oxide solid electrolyte.